The family of transmembrane proteins that conduct cations, Na+, K+, Ca2+, and anions, Cl−, I−, Br−, through the lipid membrane takes part in many functions fundamental to the biological cell including signal transduction. Recently, the crystal structure of the voltage dependent K+ channel, KvAP, from Aeropyrum pernix, ClC Cl− channels from Escherichia coli and Salmonella enterica serovar typhimurium and mechanosensitive MscL channel (non selective) from Mycobacterium tuberculosis were determined using X-ray crystallography, biochemical and electrophysiological methods. High resolution studies of the channels reveal the ion conductive pathways and their open and closed conformations. What remains to be shown is a molecular mechanism for how the conformational change (opening and closing) in ion channels occurs in the cell-membrane.
The fourth transmembrane helix (S4) in voltage-gated channels is the primary voltage-sensing unit responsible for the opening and the closing of the channels. In molecular models such as the ‘helical screw’ or ‘paddle’ models, channels are opened and closed by a large translocation of the S4 segment in which the charged S4 residues travel across the full thickness of the lipid membrane. For example, in the paddle model, the S4 segment is buried in the lipid bilayer and traverses approximately 20 Å across the bilayer. However, accessibility data, resonance energy transfer and potentiometric studies suggest that the S4 residues move relatively small distances. Recently, it was shown that, during the gating process, the main voltage sensing element (S4) undergoes only a small movement relative to the membrane plane. It is important to note that, in all current models, membrane lipids hosting the channel proteins are considered to be dielectric and their structure is considered to play only a passive role in the conformational changes in these channel proteins.
The structure of the cell membrane in the generally accepted fluid mosaic model, a two dimensional smectic liquid crystal bilayer comprising amphiphilic lipids, cholesterol and embedded proteins, is considered to be a dielectric and to play only a passive role in the activities of the biological cell. However, there is strong evidence pointing toward an active role of the membrane in a wide range of signal transductions of the cell that can arise only from structural transition of the membrane lipids.
It has recently been observed that the amphiphilic lipid molecules are tilted in their bilayers. The tilted amphiphilic lipid layers are electrically polarized. In view of this I have made the following observations. An electric pulse along the plane of the bilayer can cause reorientation of spontaneous polarization accompanied by collective rotation of the lipid molecules. In the cell membrane, this in-plane electric pulse arises, for example, from charged residues of the transmembrane proteins. This dynamic property of the lipid membranes can be linked to signal transduction of proteins and can be investigated by applying an external in-plane electric field to the cell membrane.
In a bilayer, an in-plane component of the permanent dipole moment of one layer makes an angle with its counterpart of the opposing layer when both layers have the same tilt direction. The resulting dipole moment lies perpendicular to the tilt direction of the double layer producing a net polarization. These dipole moments can be realigned by an external electric field in a collective rotation of the molecules around a cone determined by the tilt angle of the molecules. The collective rotation of lipid molecules in a cone and the reorientation of the polarization in the lipid bilayers in cell membranes can have several effects. This can cause reversible conformational changes in membrane bound proteins and vice versa because anisotropic liquid crystals can transmit torque over a macroscopic distance. This can be expected because proteins which can be dissolved in amphiphilic lipids without disrupting the phase structure of the lipids may also exist in liquid crystalline state and it is likely that there is hydrogen bonding between proteins and the lipid head groups. The link between this reversible dynamic structural behavior of the membrane lipids and the conformational transition of the membrane bound proteins may potentially be common to all signal transductions in the biological cell.
No device has been developed to permit probing structural changes of the membrane by applying an in-plane electric field. A widely used method of preparing both symmetric and asymmetric planar membranes suitable for electrical measurements has been developed by Montal and Mueller in which an electric field can be applied perpendicular to the membrane. In this method, the bilayers are formed as a film in a small aperture on a thin Teflon septum placed between two chambers containing monolayers in an aqueous solution. The structure of the membrane formed in this apparatus is known only in the middle of the film and the structure around the vicinity of septum is not well defined. Besides this, placing the electrode on the septum is technically extremely difficult. Therefore, this method cannot be extended or modified for application of an in-plane electric field.